First slide

Radiation

Question

A black body at $227^{0} C$ radiates heat at the rate of 7 cal ${cm}^{- 2} s^{- 1}$ . At a temperature of $727^{0} C,$ the rate of heat radiated in the same units will be

Easy

A

60
B

50
C

112
D

80

Solution

Apply Stefan's law of radiation, E = ${σT}^{4}$
$\Rightarrow \frac{E_{1}}{E_{2}} = {[\frac{T_{1}}{T_{2}}]}^{4}$
$\Rightarrow E_{2} = 7 {[\frac{273 + 727}{273 + 227}]}^{4} = {(\frac{1000}{500})}^{4} \times 7$
$\Rightarrow E_{2} = 112 cal - {cm}^{- 2} s^{- 1}$

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